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${! #} enter the last parameter

$# number of input parameters

$* enter all parameter strings and treat all parameters as one string.

$n output n parameters

$? The exit status of the previous command, or the return value of the function.

$the current Shell process ID. For Shell scripts, it is the process ID where these scripts are located.

The difference between $* and $@

Both $* and $@ indicate that all arguments passed to a function or script, when not contained in double quotes (""), are marked with "$1"$2". Outputs all parameters in the form "$n".

But when they are enclosed in double quotes (""), "$*" takes all the parameters as a whole and outputs all parameters in the form of "$1 $2 … $n"; "$@" separates the parameters as "$1"$2"... Outputs all parameters in the form "$n".

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